1. Field of the Invention
The present invention relates to an electroplating bath for forming a zinc-nickel alloy coating on an electrically conductive metallic matrix and method for electroplating the same, more particularly to an electroplating bath and electroplating method employing an alkaline electroplating solution in which any cyanide is not included.
2. Description of the Prior Art
There has been known that zinc-nickel alloy is electroplated on metallic products in order to improve the corrosion resistance property thereof.
Conventionally, the zinc-nickel alloy electroplating process has been carried out in an acid electrolytic bath such as sulfate bath, chloride bath and sulfamate bath.
For example, in Japanese Patent Publication No. 58-39236, there is shown a method for electroplating the zinc-nickel alloy by employing an acid bath. In the case of the acid bath, it is advantageous that a high current efficiency can be obtained and the electroplating can be carried out under a high current density to reduce the processing time. This method can be effectively applied to products having a simple configuration such as steel plate and wire. However, for products having a complex configuration such as pressed products, welded parts and piping parts, the method employing the acid bath is disadvantageous in the fact that it is impossible to obtain a desirable homogeneity with regard to the thickness of the zinc-nickel alloy coating formed on the surface of the metallic products and a zinc-nickel alloy ratio which indicates a ratio of zinc and nickel contained in the coating.
There is shown an electroplating method employing a neutral bath in Japanese Patent Public Disclosure No. 59-185792. The method employing the neutral bath can improve the property of the zinc-nickel alloy coating formed on the product surface in comparison with the method employing the acid bath. There, however, occurs a problem that a large amount of complexing agent is needed for dissolving zinc and nickel so that a sophisticated drain processing unit is necessary for treating the complexing agent. In addition, it is disadvantageous in the fact that the solution in the bath becomes unstable because of the complexing agent.
In the method employing the neutral bath, a lot of chloride is commonly added in order to improve electrical conductivity. However, chloride has a high corrosive property so that the electroplating equipment and the electroplated product itself may be subjected to a corrosive action of chloride.
Further, there is shown an electroplating method employing an alkaline bath in Japanese Patent Public Disclosure No. 51-28533. In this method, a low corrosive electrolytic bath can be employed for electroplating. Therefore, it is advantageous in terms of the cost of the electroplating equipment in comparison with the methods employing the acid and neutral baths.
There, however, occurs a problem in the method employing the alkaline bath as disclosed in the Japanese Patent Public disclosure No. 51-28533 that cyanide is inevitably added for the alkaline bath so that the special drain processing unit has to be introduced for processing the cyanide contained in the solution because of high toxicity thereof. In addition, the working atmosphere is harmed.